Materials used as intermediates for preparing drugs, insecticides, agricultural chemicals, cosmetics, food nutrients, additives, perfumes and compounds often exist only as a chiral pair. However, only one isomer of the chiral pair shows the activity of acting as a useful material, and the other isomer is inactive or even shows toxicity. Thus, currently, there is a continued need to obtain optically pure enantiomers. Also, researchers in the academic or industrial fields now concentrate much effort either on finding more reliable and effective reaction mechanisms for synthesizing optically pure compounds or on separating useful active materials from inactive or toxic compounds.
Meanwhile, porous materials with large surface area, which indicate metal oxide, metal salt or organic-metal coordination polymer compounds or materials containing organic network structures, are currently known to play an important role in the recognition and adsorptive transport of molecules required in biological procedures and industrial processes, and the separation of gaseous molecules [Davis, M. E. Davis Nature 2002, 417, 813; Kesanli B.; Lin W., Coordination Chemistry Reviews, 2003, 246, 305].
A typical example of these porous materials is zeolite, a porous solid oxide consisting of aluminum and silicon. Zeolite has cavities and tunnels with regular open structures and is excellent in durability, reproducibility and thermal stability, and its physical and chemical properties can be controlled due to its structural characteristics. Thus, it has been used mainly in wide applications, including catalysts, adsorbents for separation, and gas storage containers. Also, zeolite has advantages in that it makes ion exchange processes very easy, is inexpensive, and causes little or no by-products. Accordingly, scientists now make efforts to develop not only novel uses of zeolite, including applications to chemical or electrical sensors or selective membranes, and stationary phases for high-resolution liquid chromatography, but also new zeolite materials with larger cavities and tunnels.
Even though porous materials including zeolite have many structural advantages as described above, only a very small number of chiral porous framework materials were prepared.
For example, protein crystals crosslinked so as to have chiral characteristics inherent in protein molecules while maintaining high porosity were prepared, and their use allowed highly successful separation of racemic compounds [Vilenchik, Lez Z.; Griffith, J. P.; Clair, N. St.; Navia, M. A.; Margolin, A. L. J. Am. Chem. Soc. 1998, 120, 4290]. Also, metal organic framework(MOF)-type solids were prepared by coordinating metal ions to chiral organic linkage units, and these solids have open-framework structures with chiral cavities and tunnels and were seen to be sufficiently applicable as catalysts or adsorbents in the selective synthesis of isomers [Kepert, C. J.; Prior, T. J.; Rosseinsky, M. J. J. Am. Chem. Soc. 2000, 122, 5158; Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim, K. Nature 2000, 404, 982]. However, the highly porous organic or organic-metal framework structures as described above have disadvantages in that they require many costs for preparation, have a weak framework structure due to hydrogen bonds or nonbonded interactions caused by the overlapping of π-π electron clouds, and have poor thermal stability.
Unlike these organic or organic-metal framework structures, in the case of zeolite, a combination of large surface area and phase caused by strong covalent bonds between Si—O and Al—O is seen to be a factor of determining isomer selectivity and durability. However, since preparing chiral zeolite directly from metal oxide encounters limitations, there is only a very small number of reports on the selective synthesis or separation of isomers by chiral zeolite.
For example, there are reports on the synthesis and separation of isomers by aluminosilicate zeolite β and titanosilicate ETS-10. However, these two zeolite materials have very dense crystals grown therein, and thus, maintain pure properties of isomers only at very small crystal lattice layers [Treacy, M. M. J.; Newsam, J. M. Nature 1988, 332, 249; Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Philippou A.; Mackay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Nature 1994, 367, 347].
For these reasons, attempts are now made to prepare inorganic-organic composite porous materials by binding or incorporating chiral organic molecules into porous inorganic materials other than making zeolite-like porous materials themselves chiral. The preparation of the inorganic-organic composite porous materials has received a great interest, since it is performed in various operational conditions as compared to the case of organic polymers in view of the selection of temperature or solvents, etc. Here, as the inorganic supports for binding and incorporating the chiral molecules, zeolites have been used.
As a modified method using zeolite, a method of binding chiral molecules to silane groups on the zeolite surface by covalent bonds has been widely adopted. This method provided an opportunity to synthesize inorganic-organic composite materials. There are several reports that non-chiral zeolites or zeolite-like materials were chiral-functionalized by this method.
Mezoporous silica material M41S, which can be used as a stationary phase for high-performance liquid chromatography (HPLC), is a modified material obtained by covalently binding chiral molecules to a non-chiral surface. This modified material appeared to be a successful solution but showed very low efficiency [Thoelen, C.; van der Walle, K.; Vankelecom, I. F. J.; Jacobs, P. A. Chem. Commun. 1999, 184]. In addition, there is a report on a chiral inorganic-organic composite catalyst obtained by covalently binding transition metal complexes or organic molecules to zeolite or silica. The physical stability of this catalyst was actually improved but it had no enantiomer selectivity [Alcn, M. J.; Corma, A.; Iglesias, M.; Snchez, F. J. Mol. Cat. A: Chem. 2003, 194, 137]. Furthermore, there was chiral Y-zeolite which is a chiral-selective catalyst acting in a solution-phase photoreaction producing a racemic mixture. This chiral-selective catalyst was obtained by coordinating chiral molecules to charge-balancing metal cations and showed a chiral selectivity of more than 90% [Chong, K. C. W.; Sivaguru, J.; Shichi, T.; Yoshimi, Y.; Ramamurthy, V.; Scheffer, J. R. J. Am. Chem. Soc. 2002, 124, 2858].
However, the above-described methods, i.e., the method of covalently binding chiral molecules to the zeolite surface or the method of coordinating chiral molecules to charge-balancing metal cations, encounter a difficulty in selecting a solvent and have the limitation of insufficient structural stability, resulting in the leakage of chiral molecules into solution.
Accordingly, there is now a need for the development of chiral porous materials which can be used, for example, as chiral-selective catalysts or materials of separating isomeric mixtures.